Fuel cell power plants are well-known and are commonly used to produce electrical energy from reducing and oxidizing fluids to power electrical apparatus such as apparatus on-board space vehicles. In such power plants, a plurality of planar fuel cells are typically arranged in a stack surrounded by an electrically insulating frame structure that defines manifolds for directing flow of reducing, oxidant, coolant and product fluids. Each individual cell generally includes an anode electrode and a cathode electrode separated by an electrolyte. A fuel such as hydrogen or methanol is supplied to the anode electrode, and an oxidant such as oxygen or air is supplied to the cathode electrode. In a cell utilizing a proton exchange membrane as the electrolyte, the fuel electrochemically reacts at a surface of the anode electrode to produce hydrogen ions and electrons, and carbon dioxide if the fuel is methanol. The electrons are conducted to an external load circuit and then returned to the cathode electrode, while the hydrogen ions transfer through the electrolyte to the cathode electrode, where they react with the oxidant and electrons to produce water and release thermal energy.
The anode and cathode electrodes of such fuel cells are separated by different types of electrolytes depending on operating requirements and limitations of the working environment of the fuel cell. One such electrolyte is a proton exchange membrane ("PEM") electrolyte, which consists of a solid polymer well-known in the art. Other common electrolytes used in fuel cells include phosphoric acid or potassium hydroxide held within a porous, non-conductive matrix between the anode and cathode electrodes. It has been found that PEM cells have substantial advantages over cells with liquid acid or alkaline electrolytes in satisfying specific operating parameters because the membrane of the PEM provides a barrier between the reducing fluid and oxidant that is more tolerant to pressure differentials than a liquid electrolyte held by capillary forces within a porous matrix. Additionally, the PEM electrolyte is fixed, and cannot be leached from the cell, and the membrane has a relatively stable capacity for water retention. As is well-known however, PEM cells have significant limitations especially related to liquid water transport to, through and away from the PEM, and related to simultaneous transport of gaseous reducing and oxidant fluids to and from the electrodes adjacent opposed surfaces of the PEM. The prior art includes many efforts to minimize the effect of those limitations.
In operation of a fuel cell employing a PEM, the membrane is saturated with water, and the anode electrode adjacent the membrane must remain wet. As hydrogen ions produced at the anode electrode transfer through the electrolyte, they drag water molecules in the form of hydronium ions with them from the anode to the cathode. Water also transfers back to the anode from the cathode by osmosis. Product water formed at the cathode electrode is removed by evaporation or entrainment into a circulating gaseous stream of oxidant, or by capillary action into and through a porous fluid transport layer adjacent the cathode.
It is known to utilize an aqueous alcohol solution or mixture as a fuel for fuel cells having PEM electrolytes, such as an aqueous methanol fuel. For purposes herein, the phrase "aqueous alcohol" is to be understood to mean a solution or mixture including water and methanol, ethanol, propanol, butanol or pentanol or combinations thereof. An advantage of an aqueous alcohol fuel is that a risk of drying out of the PEM adjacent the anode electrode is virtually eliminated because a liquid mixture of alcohol and water is supplied directly to the anode electrode. Another advantage is that, by directly oxidizing the alcohol at the anode electrode, there is no need for fuel processing components that are normally used in association with fuel cells to process organic fuels into hydrogen rich fuel streams. As disclosed in U.S. Pat. No. 5,573,866 issued on Nov. 12, 1996 to Van Dine et al. and assigned to the assignee of the invention disclosed herein, which patent is hereby incorporated herein by reference, direct aqueous alcohol fueled fuel cells may be supplied with a solution containing between about 1% to about 65% alcohol by weight with the remainder of the solution being water.
Direct aqueous alcohol fuel cells include at least two fuel cell configurations that utilize aqueous alcohol as a fuel source that is directly exposed to an anode electrode within the cell. For purposes of consistency herein, a first type of direct aqueous alcohol fuel cell will be characterized as a split reactant fuel cell, such as the fuel cell described in the aforesaid U.S. Pat. No. 5,573,866 to Van Dine et al. A second type of direct aqueous alcohol fuel cell will be characterized for purposes herein as a mixed reactant fuel cell.
In a split reactant fuel cell, an aqueous alcohol fuel and gaseous oxidant reactants are separated. The aqueous alcohol fuel is directed into contact exclusively with an anode electrode, while the oxidant is directed into contact with the cathode electrode. Typically the aqueous alcohol fuel is passed through carbon plates that have transverse grooves adjacent the anode electrode, or through porous plates adjacent the anode electrode while the gaseous stream of oxidant passes through porous or grooved plates adjacent the cathode electrode to supply oxidant to the cathode and to sweep away by evaporation and/or entrainment water formed at the cathode and any cooling water directed to the cathode electrode. The water formed at the cathode electrode and the cooling water may also be moved away from the electrode by capillary action into and through a porous fluid transport layer adjacent the cathode in a manner well-known in the art.
In a mixed reactant fuel cell, an aqueous alcohol fuel is mixed with a gaseous oxidant stream such as air and the alcohol, water and air are simultaneously directed past both the anode and cathode electrode. In such a mixed reactant fuel cell, the anode and cathode electrode configurations are fabricated to favor oxidation of the fuel at the anode electrode and reduction of the oxidant at the cathode electrode in a manner well-known in the art. Examples of mixed reactant fuel cells are disclosed in an article written by C. K. Dyer, entitled "A Novel Thin-Film Electrochemical Device for Energy Conversion" and in an article written by T. E. Mallouk entitled "Miniaturized Electrochemistry", both articles being published in "NATURE--International Weekly Journal of Science", Vol. 343, No. 6258, dated Feb. 8, 1990 at Pages 547 and 515 respectively, which articles are hereby incorporated herein by reference.
Both split and mixed reactant types of direct aqueous alcohol fuel cells suffer from major problems related to aqueous alcohol being adjacent the cathode electrodes. Split reactant fuel cells experience alcohol cross-over through the PEM electrolyte. Known PEM electrolytes are permeable to both water and alcohol, and consequently some alcohol crosses over from the anode side to emerge adjacent the cathode electrode to be swept out of the fuel cell in a cathode exhaust stream along with the oxidant stream passing the cathode electrode. Mixed reactant fuel cells direct aqueous alcohol adjacent the cathode electrode along with the gaseous oxidant.
A first major problem of such direct aqueous alcohol fuel cells therefore is that common platinum-based cathode electrodes in the presence of oxygen and alcohol normally oxidize the alcohol, for by example oxidizing methanol in a reaction that produces carbon dioxide and water with the energy of the reaction producing heat instead of electrical energy. Not only would the additional heat reduce the efficiency of the fuel cell by increasing necessary cooling requirements, but also the alcohol is lost as a fuel source to produce electrical energy. To prevent oxidation of alcohol at cathode electrodes in direct aqueous alcohol fuel cells, special cathode electrodes have been developed from catalytic compositions that do not oxidize alcohols such as methanol to a significant degree. Such cathode electrodes for direct aqueous alcohol fuel cells are known in the art and are described as applied to methanol for example in the aforesaid Patent to Van Dine, as well as in an article written by V. Trapp, P. Christensen, and A. Hamnett, entitled "New Catalysts for Oxygen Reduction Based on Transition-Metal Sulfides", and published in J. Chem. Soc., Faraday Trans., 1996, Vol. 92, at Pages 4311-4319. For purposes of consistency, such cathode electrodes that do not oxidize alcohol to a significant degree will be referred to hereinafter as "selective cathode electrodes".
A second problem associated with having aqueous alcohol adjacent the cathode electrodes is that, even if selective electrodes prevent oxidation of the alcohol, it is nonetheless swept into the cathode exhaust stream and thereby removed from the fuel cell. One solution to that problem has been to utilize condensing heat exchangers exposed to ambient air to cool and thereby condense evaporated alcohol and water, and thereafter direct the condensed alcohol and water back into the fuel cell, as disclosed in Van Dine for a split reactant direct aqueous alcohol fuel cell using an aqueous methanol fuel. Such condensing heat exchangers, however, encounter decreasing efficiencies as ambient temperatures increase. Where the power plant is to power a transportation vehicle such as an automobile, the plant will be exposed to an extremely wide range of ambient temperatures. For example where an ambient air coolant stream passes through a heat exchanger, performance of the exchanger in recovering alcohol and water will vary as a direct function of the temperature of the ambient air because decreasing amounts of liquid precipitate out of power plant exhaust streams as the ambient air temperature increases. Additionally, condensing heat exchangers typically require fan apparatus and related manifolding, switching and controls to maintain proper passage of the ambient air through the exchanger, thereby adding weight, cost and complexity to such a fuel cell power plant. Therefore it is desirable to develop a direct aqueous alcohol fuel cell power plant that enhances recovery of alcohol and water in plant exhaust streams while decreasing weight, complexity and cost of the plant.